Optical scanning apparatus, image forming apparatus, and support member for an optical scanning apparatus

ABSTRACT

Provided is an optical scanning apparatus, including: an optical deflector having a polygon mirror reflecting a light flux emitted from a light source to carry out scanning, a drive unit rotating the polygon mirror about a rotation shaft, and a board supporting the drive unit; an optical box to which the board is fixed; two holes provided in the optical box, through which two screws fixing the board are inserted, respectively; two bearing surfaces provided on the optical box at positions around the two holes, respectively, the two bearing surfaces supporting the board; and an abutment portion provided on the optical box and abutting against a portion that is prevented from being pressed by the two screws in the board which is sandwiched between the two screws and the two bearing surfaces so as to be fixed to the optical box.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical scanning apparatus forcarrying out optical writing on a photosensitive drum or the like withuse of a laser beam and a support member for an optical scanningapparatus in an image forming apparatus such as a laser beam printer(hereinafter referred to as “LBP”), a digital copying machine, and adigital facsimile machine.

2. Description of the Related Art

Image forming apparatus such as an LBP is configured to form an image ona recording material by forming an electrostatic latent image on aphotosensitive drum in accordance with a desired image, developing theelectrostatic latent image into a toner image, and by transferring thetoner image onto the recording material. To form a desired electrostaticlatent image on the photosensitive drum, there is used an opticalscanning apparatus for irradiating and scanning the photosensitive drumwith a laser beam.

Therefore, the print accuracy of such an image forming apparatus issignificantly affected by the mechanical accuracy of the opticalscanning apparatus. In particular, in an optical deflector forrotationally driving a rotating polygon mirror, the rotation center ofthe rotating polygon mirror, which serves as a reference of an opticalsystem of the LBP, needs to be positioned with respect to an opticalreference position of an optical box with high accuracy.

In view of the above, there is generally employed such positioningstructure that a shaft or bearing of the optical deflector is fittedinto a fitting hole provided at the optical reference of the opticalbox.

Conventionally, an optical deflector including a polygon mirror and amotor unit for rotating the polygon mirror is generally fixed withscrews at three points surrounding the motor. In this case, a board isfixed in conformity to bearing surfaces at the respective screw fixingportions. Therefore, when the bearing surfaces are inclined, the boardis distorted and the shaft of the motor unit of the optical deflector isinclined.

Further, the inclination of the rotation shaft of the optical deflector,which is called shaft inclination, may cause distortion of a spot-shapeof the laser beam on the photosensitive drum, resulting in deteriorationin image quality.

According to Japanese Patent Application Laid-Open No. 2005-201941, theoptical deflector is fixed to the optical box with use of a screw and aspring, and the screw is tightened and loosened after the opticaldeflector is fixed, to thereby adjust the push-in amount and control theshaft inclination.

In the structure disclosed in Japanese Patent Application Laid-Open No.2005-201941, however, the assembly process becomes complicated for thesake of shaft inclination control, thus leading to a problem in that thenumber of steps is increased.

SUMMARY OF THE INVENTION

The present invention provides a structure for suppressing shaftinclination of an optical deflector even without a shaft inclinationcontrol function.

The present invention provides an optical scanning apparatus, including:an optical scanning apparatus, including: an optical deflector having apolygon mirror reflecting a light flux emitted from a light source tocarry out scanning, a drive unit rotating the polygon mirror about arotation shaft, and a board supporting the drive unit; an optical box towhich the board is fixed; two holes provided in the optical box, throughwhich two screws fixing the board are inserted, respectively; twobearing surfaces provided on the optical box at positions around the twoholes, respectively, the two bearing surfaces supporting the board; andan abutment portion provided on the optical box and abutting against aportion that is prevented from being pressed by the two screws in theboard which is sandwiched between the two screws and the two bearingsurfaces so as to be fixed to the optical box.

Moreover, the present invention provides an image forming apparatus,including: a photosensitive member; an optical scanning apparatusirradiating and scanning the photosensitive member with light, theoptical scanning apparatus having an optical deflector and an opticalbox; the optical deflector including a polygon mirror reflecting a lightflux emitted from a light source to carry out scanning, a drive unitrotating the polygon mirror about a rotation shaft, and a boardsupporting the drive unit; the optical box to which the board is fixed;a developing device forming a toner image by causing toner to adhereonto the photosensitive member that is irradiated with the light by theoptical scanning apparatus; a transfer device transferring the tonerimage, which is formed by the developing device, onto a recordingmaterial; two holes provided in the optical box, through which twoscrews fixing the board are inserted, respectively; two bearing surfacesprovided on the optical box at positions around the two holes,respectively, the two bearing surfaces supporting the board; and anabutment portion provided on the optical box, and abutting against aportion that is prevented from being pressed by the two screws in theboard which is sandwiched between the two screws and the two bearingsurfaces so as to be fixed to the optical box.

Moreover, the present invention provides a support member for an opticalscanning apparatus, which is configured to support an optical deflectorhaving a polygon mirror reflecting a light flux emitted from a lightsource to carry out scanning, a drive unit rotating the polygon mirrorabout a rotation shaft, and a board supporting the drive unit, thesupport member including: two holes through which two screws fixing theboard are inserted, respectively; two bearing surfaces provided atpositions around the two holes, respectively, the two bearing surfacessupporting the board; and two abutment portions abuttable againstportions that are prevented from being pressed by the two screws in theboard, one of the two abutment portions abutting against the board whenthe board of the optical deflector is sandwiched between the two screwsand the two bearing surfaces so as to be fixed to an optical box.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating the structure of an opticalscanning apparatus according to an embodiment of the present invention.

FIG. 2 is a sectional view illustrating an optical deflector of theoptical scanning apparatus in FIG. 1.

FIG. 3 illustrates assembly of the optical deflector according to theembodiment of the present invention.

FIG. 4 is a perspective view illustrating a state in which the opticaldeflector is assembled to an optical box in a case where abutmentportions are not provided.

FIG. 5 is a sectional view illustrating a state in which the opticaldeflector is fastened to the optical box with screws in the case wherethe abutment portions are not provided.

FIG. 6A is a perspective view illustrating a state in which the opticaldeflector is fastened to the optical box.

FIG. 6B is a perspective view illustrating a state of deformationoccurring when the optical deflector is fastened to the optical box inthe case where the abutment portions are not provided.

FIG. 7 is a perspective view illustrating a state in which the opticaldeflector is fastened with screws to an optical box having one bearingsurface inclined toward the other bearing surface in the case where theabutment portions are not provided.

FIG. 8 is a side view illustrating deformation of a board in the case ofFIG. 7.

FIG. 9 is a scatter plot of angles of shaft inclination of a rotationshaft in the case where the abutment portions are not provided.

FIG. 10 is a perspective view of the optical scanning apparatus, forillustrating a state in which the abutment portions suppress thedeformation of the board according to the embodiment of the presentinvention.

FIG. 11 is a scatter plot of angles of shaft inclination of a rotationshaft according to the embodiment of the present invention.

FIG. 12 illustrates the structure of an image forming apparatusaccording to the embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Now, an embodiment of the present invention is described in detail withreference to the attached drawings.

(Overall Structure of Image Forming Apparatus)

FIG. 12 illustrates the overall structure of an image forming apparatusaccording to the embodiment of the present invention. The image formingapparatus is a printer that employs an electrophotographic process.

A printer 100 is configured to receive image information from anexternal host computer or the like (not shown), and to record an imagein accordance with the received image information by forming the imageonto a sheet-like recording material P.

When the printer 100 receives a print signal, the printer 100 startsrotational drive of a photosensitive drum 111 (object to be irradiated,or image bearing member) serving as a photosensitive member for bearinga toner image. The photosensitive drum 111 is rotationally driven in aclockwise direction at a predetermined peripheral speed. A chargingroller 112 having a predetermined bias applied thereto charges thesurface of the photosensitive drum 111 at a predetermined potential.

Subsequently, in accordance with the image information received from thehost computer or the like (not shown), an optical scanning apparatus 113scans and exposes with light the charged portion on the surface of thephotosensitive drum 111. In the region exposed with light, anelectrostatic latent image is formed on the surface of thephotosensitive drum 111 in accordance with the image information. Adeveloping device 114 (developing unit) causes toner to adhere onto thesurface of the photosensitive drum 111, and thus the electrostaticlatent image is visualized as (developed into) a toner image.

A driven feed roller 115 separates and feeds a single recording materialP from a feed cassette at a predetermined timing. The recording materialP fed from the feed cassette is fed by a registration roller pair 116 ata predetermined control timing to a transfer nip portion NIP formedbetween the photosensitive drum 111 and a transfer roller 117 (transferunit). In the process in which the recording material P is nipped andconveyed through the transfer nip portion NIP, the toner image on thephotosensitive drum 111 is transferred onto the recording material P ina sequential manner.

After the toner image is transferred, a fixing device 118 heats andfixes the toner image onto the recording material P. Then, the recordingmaterial P passes through a delivery roller pair 119, and is deliveredoutside the printer main body.

A cleaning device 120 removes untransferred toner remaining on thephotosensitive drum 111, and the series of image forming processes iscompleted.

(Structure of Optical Scanning Apparatus)

FIG. 1 is a perspective view illustrating the structure of the opticalscanning apparatus 113. In front of a semiconductor laser unit 2 (lightsource), a cylindrical lens 4, an optical stop 6, and an opticaldeflector 9 are arranged in sequence. The optical deflector 9 isconfigured to rotationally drive a rotating polygon mirror 7. In adirection of light reflection of the rotating polygon mirror 7, an fθlens 11 and the photosensitive drum 111 are arranged in sequence.

An optical box 5 serves as a casing for receiving those optical members,and also serves as a support member for supporting the optical deflector9 as described later. The optical members are received in a spaceenclosed by the optical box 5, a lid 13, and the like.

A laser beam L (light flux) emitted from the semiconductor laser unit 2forms a linear image on a reflection surface 8 of the rotating polygonmirror 7 through the cylindrical lens 4. The laser beam L is deflectedthrough rotation of the rotating polygon mirror 7 by the opticaldeflector 9, and forms an image on the photosensitive drum 111 throughthe fθ lens 11 to irradiate and scan the photosensitive drum 111.

Through the rotation of the rotating polygon mirror 7, main scanning ofthe photosensitive drum 111 with the laser beam L is carried out, andthrough the rotational drive of the photosensitive drum 111 about anaxis of its cylinder, sub-scanning of the photosensitive drum 111 withthe laser beam L is carried out. In this manner, the electrostaticlatent image is formed on the surface of the photosensitive drum 111.

(Structure of Optical Deflector)

FIG. 2 is a sectional view illustrating the structure of the opticaldeflector 9 for rotationally driving the rotating polygon mirror 7.

A rotor unit 20 (rotational drive unit) of the optical deflector 9includes a rotation shaft 21, a flange member 22, the rotating polygonmirror 7, a rotor yoke 23, and a rotor magnet 24. The rotation shaft 21is integrally coupled to the rotor yoke 23 through an intermediation ofthe flange member 22, and the rotor magnet 24 is integrally coupled tothe rotor yoke 23. The rotating polygon mirror 7 is fixed to the flangemember 22. That is, the rotation shaft 21, the flange member 22, therotor yoke 23, the rotor magnet 24, and the rotating polygon mirror 7are integrally coupled to each other, and those members are integrallyrotated when the rotation shaft 21 is rotated.

A board 28 and a sleeve 29 are integrally coupled to each other, and thesleeve 29 supports the rotor unit 20 in a rotatable manner through anintermediation of the rotation shaft 21.

A stator core 31 including a stator coil 30 is also fixed to the board28 integrally. The board 28 is a circuit board made of iron, andelectronic components such as an integrated circuit for driving therotating polygon mirror 7 are mounted on the board 28. The board 28 isnot limited to the circuit board made of iron, and may be a printedboard such as a phenolic-paper board and an epoxy-glass board combinedwith a metal plate.

In the optical deflector 9, a drive circuit (not shown) sequentiallyswitches currents flowing through the stator coil 30, to therebyrotationally drive the rotor unit 20. That is, through the switching ofcurrents flowing through the stator coil 30, a magnetic field generatedby the stator coil 30 and the stator core 31 is changed. As a result, arotational force is applied to the rotor magnet 24, and thus the rotorunit 20 is rotated.

(Assembly of Optical deflector to Optical Box)

FIG. 3 is a perspective view of the optical scanning apparatus 113according to this embodiment, for illustrating a state in which theoptical deflector 9 is assembled to the optical box 5.

Screw holes 43 a and 43 b are provided in the board 28 of the opticaldeflector 9 at positions in the vicinity of the rotation shaft 21.Bearing surfaces 42 a and 42 b (bearing surfaces for supporting theboard 28) are provided on the optical box 5 at positions around holesthrough which screws 41 a and 41 b are inserted to fix the board 28,respectively. Abutment portions 45 and 46 are provided on the opticalbox 5 at positions other than the positions of the bearing surfaces 42 aand 42 b. As illustrated in FIG. 3, the bearing surfaces 42 a and 42 band the abutment portions

-   and 46 are provided so as to protrude from the bottom surface of the    optical box 5 toward the board 28 (toward the board). A straight    line F connecting the bearing surfaces 42 a and 42 b and a line    segment E passing through the abutment portions 45 and 46 are    substantially orthogonal to each other at the center of a fitting    hole 44. That is, the abutment portion 45 and the abutment portion    46 are arranged opposite to each other across the straight line F.    As is also understood from FIG. 3, the center of the fitting hole 44    is arranged on a line segment corresponding to a section between the    bearing surfaces 42 a and 42 b in the straight line F connecting the    bearing surfaces 42 a and 42 b.

The position of the optical deflector 9 is determined by fitting thesleeve 29 into the fitting hole 44 that is provided in the optical box5. The optical deflector 9 is fixed in the following manner. That is,portions around the screw holes 43 a and 43 b of the board 28 arepressed by the screws 41 a and 41 b toward the bearing surfaces 42 a and42 b of the optical box 5, and the portions around the screw holes 43 aand 43 b of the board 28 are sandwiched between the screws 41 a and 41 band the bearing surfaces 42 a and 42 b, respectively, so that the board28 is fastened. When the optical deflector 9 is fastened with the screws41 a and 41 b, the board 28 may be deformed due to inclination of thebearing surfaces 42 a and 42 b. That is, when the board 28 is fastenedto the bearing surfaces 42 a and 42 b with the screws 41 a and 41 b,respectively, the portions around the screw holes 43 a and 43 b of theboard 28 are pressed by the screws 41 a and 41 b. Thus, the portionsaround the screw holes 43 a and 43 b of the board 28 are brought intocontact with at least a plurality of points on the screws 41 a and 41 band the bearing surfaces 42 a and 42 b, or are substantially broughtinto surface contact with the screws 41 a and 41 b and the bearingsurfaces 42 a and 42 b. As a result, the board 28 is deformed so as tosubstantially follow a tilt direction (inclination) of the bearingsurfaces 42 a and 42 b.

When the optical deflector 9 is fastened with the screws 41 a and 41 b,the abutment portions 45 and 46 abut against and support portions of theboard 28 that are not pressed by the screws 41 a and 41 b. Thus, theboard 28 is pushed back, to thereby suppress the deformation of theboard 28 that may occur due to the inclination of the bearing surfaces42 a and 42 b.

That is, the abutment portions 45 and 46 are abuttable against thebottom surface of the board 28 of the optical deflector 9, and when theoptical deflector 9 is fixed to the optical box 5 with the rotationshaft 21 inclined with respect to the proper mounting direction, theabutment portions 45 and 46 abut against the optical deflector 9 tosuppress the inclination.

The abutment portions 45 and 46 are abuttable against the board 28 ofthe optical deflector 9, but unlike the bearing surfaces 42 a and 42 b,the board 28 is not fixed with screws or the like at the abutmentportions 45 and 46. That is, the abutment portions 45 and 46 abutagainst the portions of the board 28 that are not pressed by the screws.Therefore, even when the board 28 abuts against the abutment portions 45and 46, the board 28 is not twisted as compared to the case where theboard 28 is pressed by the screws at the abutment portions 45 and 46.However, when the board 28 abuts against the abutment portions 45 and46, the board 28 is substantially brought into contact with the abutmentportions 45 and 46 only at a single point. It is herein assumed that the“abutment portion” is a portion without the screw fastening or the likebetween the abutment portion and the object (board 28) subjected toabutment. As long as the board 28 is substantially brought into contactwith the abutment portions 45 and 46 only at a single point, there maybe provided such an elastic member that the portions of the board 28that abut against the abutment portions 45 and 46 are pressed toward theabutment portions 45 and 46 under a small pressure.

Next, a state in which shaft inclination of the optical deflector 9occurs when the bearing surfaces of the optical box 5 are inclined isdescribed.

FIG. 4 is a perspective view illustrating a state in which the opticaldeflector 9 is assembled to an optical box 5. FIG. 4 illustrates a crosssection cut along a plane passing through the center of the bearingsurface 42 b of the optical box 5. Referring to FIG. 4, description isgiven by way of the optical box 5 that does not include the abutmentportions 45 and 46 provided on the optical box 5 illustrated in FIG. 3,but for the sake of easy understanding of the description, the otherreference symbols are the same as those in FIG. 3.

It is originally desired that the bearing surfaces 42 a and 42 b of theoptical box 5 be formed horizontally. However, due to variation of themolding process or the like, the bearing surfaces 42 a and 42 b may beinclined at a level of several tens of micrometers. FIG. 4 illustrates astate assuming that the bearing surface 42 b is rotated about the X axisin the coordinates illustrated in FIG. 4, that is, a state in which thebearing surface 42 b is inclined to descend along the −Y direction (forthe sake of description, the inclination of the bearing surface 42 b isexaggerated).

Next, a case where the optical deflector 9 is assembled to the opticalbox 5 having such a bearing surface 42 b is described.

FIG. 5 is a sectional view illustrating the case where the opticaldeflector 9 is fastened to the optical box 5 with the screw 41 a and thescrew 41 b. When the screw 41 b is tightened to the inclined bearingsurface 42 b, a header 61 of the screw 41 b and the inclined bearingsurface 42 b sandwich the board 28 therebetween, and the board 28 isdeformed in conformity to the inclination of the bearing surface 42 b asrepresented by a board 28 a that is indicated by the broken line. As aresult, the rotation shaft 21 is inclined at an angle α.

FIGS. 6A and 6B are perspective views of the optical deflector 9, forillustrating details of the deformation of the board 28 in the casedescribed above. FIG. 6A illustrates the board 28 mounted in anon-deformed state, and FIG. 6B exaggerates the board 28 mounted in adeformed state for the sake of description. The two-dot chain line inFIG. 6B indicates the board 28 in the non-deformed state, and the solidline in FIG. 6B indicates the board 28 in the deformed state. Asdescribed above with reference to FIG. 5, the header 61 of the screw 41b and the inclined bearing surface 42 b sandwich the board 28therebetween, and hence the header 61 applies a force to a hatchedportion A in the vicinity of the screw hole 43 b. This force acts astwisting moment applied in the arrow C direction about an axis passingthrough the center of the screw hole 43 a and the center of the screwhole 43 b (virtual axis B). Due to the twist, the board 28 is deformedand the rotation shaft 21 is inclined at the angle α.

The above description is directed to the state in which the bearingsurface 42 b is rotated about the X axis, that is, the case where thebearing surface 42 b is inclined to descend along the −Y direction.Alternatively, the bearing surface 42 b may be inclined in anotherdirection, or the bearing surface 42 a and the bearing surface 42 b maybe inclined in different directions.

FIG. 7 is a perspective view illustrating a state in which the opticaldeflector 9 is fastened with the screw 41 a and the screw 41 b to theoptical box 5 having the bearing surface 42 b inclined to descend towardthe bearing surface 42 a.

For the sake of easy understanding, the optical box 5 is cut along aplane passing through the center of the bearing surface 42 a and thecenter of the bearing surface 42 b.

FIG. 8 is a side view illustrating a state in which the opticaldeflector 9 is fastened to the optical box 5 with the screw 41 a and thescrew 41 b as seen in the arrow K direction in FIG. 7.

The inclined bearing surface 42 b and the screw 41 b sandwich the board28 therebetween, and hence the board 28 is deformed. In this case,however, the bearing surface 42 a is located in a direction in which theboard 28 is to be deformed, and hence the deformation of the board 28 issuppressed. Thus, an inclination β of the rotation shaft 21 is trivialas compared to the inclination a in FIG. 5.

As described above with reference to FIGS. 4 to 8, when portions in thevicinity of the rotation shaft 21 of the optical deflector 9 are fixedwith the screws at two points, the board 28 is twisted moresignificantly about the virtual axis B (see FIG. 6B) passing through thetwo screw fixing points, whereas the board 28 is twisted lesssignificantly about an virtual axis J (see FIG. 6B) orthogonal to thevirtual axis B. Thus, the two bearing surfaces 42 a and 42 b of theoptical box 5 may be inclined in any direction, but the board 28 istwisted more significantly about the virtual axis B passing through thetwo screw fixing points.

FIG. 9 is a scatter plot of angles of the shaft inclination of therotation shaft 21 in top view of the rotation shaft 21 when the opticaldeflector 9 is fixed to the optical box with the screws several times inthe case where the abutment portions 45 and 46 are not provided.

The X and Y coordinates of the scatter plot are identical with thecoordinates illustrated in FIG. 4. In FIG. 9, the center O indicatesthat the shaft inclination is not present. Further, the point Qindicates that the upper end portion of the rotation shaft 21 is moveddue to the shaft inclination by a distance “a” in the X direction and adistance “b” in the Y direction.

As described above with reference to FIGS. 6A and 6B, it is understoodthat the inclination of the rotation shaft 21 is distributed widely inthe arrow M direction and the arrow N direction due to the twist aboutthe virtual axis B passing through the two screw fixing points.

Next, a state in which the shaft inclination is suppressed by providingthe abutment portions 45 and 46 of this embodiment is described.

FIG. 10 is a perspective view of the optical scanning apparatus 113, forillustrating the state in which the abutment portions 45 and 46 of thisembodiment suppress the deformation of the board 28.

The optical box 5 of this embodiment includes the abutment portions 45and 46 around the fitting hole 44. In a case where the bearing surface42 a or the bearing surface 42 b is inclined about an axis F passingthrough the two screw fixing points as described above, when the opticaldeflector 9 is fixed with the screws, the board 28 is twisted about thevirtual axis B. In this embodiment, however, in a case where the board28 is twisted in the arrow C direction, the abutment portion 46 pushesback a region G of the board 28 (abutment region between the abutmentportion 46 and the board 28), and in a case where the board 28 istwisted in the arrow D direction, the abutment portion 45 pushes back aregion H of the board 28 (abutment region between the abutment portion45 and the board 28). Thus, the twist of the board 28 is suppressed.Through the suppression of the twist of the board 28, it is possible tosuppress the inclination of the rotation shaft of the optical deflector9 that occurs due to the inclination of the bearing surface 42 a or 42b.

As the accuracy of the height dimensions of the abutment portions 45 and46 (in the Z direction) is higher, the variation of the shaftinclination is further suppressed. Therefore, it is preferred that thearea of the contact portion between the abutment portions 45 and 46 andthe board 28 is as small as possible. That is, the area of contact whenthe abutment portions 45 and 46 abut against the optical deflector 9 isset smaller than the area of contact between the two bearing surfaces 42a and 42 b and the optical deflector 9 when the optical deflector 9 isfixed with the screws to the two bearing surfaces 42 a and 42 b of theoptical box 5. It is ideal that each of the abutment portions 45 and 46be brought into contact (point contact) with the optical deflector 9substantially at a single point. As the area of contact of the abutmentportions 45 and 46 is smaller, the inclination of the abutment surfacesdue to the variation of the molding process or the like becomes smallerlike the bearing surfaces 42 a and 42 b, with the result that thevariation of the height dimensions is smaller as well. As the variationof the heights of the abutment portions is smaller, the variation of theangle of pushing back the board (about the virtual axis B) is smaller aswell, with the result that the variation of the shaft inclination issmaller as well.

In this embodiment, as illustrated in FIG. 10, tapered boss portions 47and 48 are provided on the bottom surface of the optical box 5, and theabutment portions 45 and 46 are provided at the center portions of thoseboss portions 47 and 48, respectively. That is, the boss portions 47 and48 are tapered so that the sectional areas are gradually reduced as thedistance from the portion in contact with the optical box 5 isincreased.

When the boss portions 47 and 48 supporting the abutment portions 45 and46, respectively, are significantly shrunk due to a force applied so asto push back the twisted board 28, the heights of the abutment portions45 and 46 vary, which also causes the variation of the shaftinclination. Therefore, it is preferred that the sectional areas of theboss portions 47 and 48 (planes perpendicular to the Z axis) be largerso as to suppress the shrinkage of the boss portions 47 and 48. Tosatisfy the above-mentioned two conditions, it is appropriate that theboss portions 47 and 48 be tapered so that the sectional areas areincreased toward the bottom.

The heights of the abutment portions 45 and 46 (degrees of protrusionfrom the bottom surface of the optical box 5 toward the board 28 in adirection along the rotation shaft 21) are set equal to each other. Whenthe board 28 is mounted in the proper mounting direction without beingtwisted, that is, without the shaft inclination of the rotation shaft21, the heights of the abutment portions 45 and 46 may be set so thatthe abutment portions 45 and 46 do not abut against the board. As longas the abutment portions 45 and 46 do not press the board, the heightsof the abutment portions 45 and 46 may be set so that the abutmentportions 45 and 46 abut against the board. Thus, when the board 28 ismounted under a state in which the rotation shaft 21 is not inclined,the abutment portions 45 and 46 do not press the board.

When the board 28 is fixed with the screws in the case where, forexample, the bearing surface 42 a or 42 b is inclined, the board 28 istwisted in conformity to the bearing surface 42 a or 42 b. However, whenthe board 28 is twisted as described above, one of the abutment portions45 and 46 that is located on the twist side abuts against the board 28to suppress the shaft inclination. At this time, the abutment portionabutting against the board 28 is pushed by the board 28 to shrink insome degree. Therefore, a gap is generated between the opposite abutmentportion and the board 28, and the board 28 is supported on the opticalbox 5 at three points, that is, one of the abutment portions 45 and 46,the bearing surface 42 a, and the bearing surface 42 b. Further, theportion of the board 28 that abuts against the abutment portion is notdirectly pressed by the screw, and hence the board is not easilydistorted.

FIG. 11 is a scatter plot of angles of the shaft inclination of therotation shaft 21 in top view of the rotation shaft 21 when the opticaldeflector 9 is fixed to the optical box 5 of this embodiment with thescrews several times in a manner similar to that in FIG. 9. In FIG. 11,the black marks indicate angles of the shaft inclination according tothe related art as illustrated in FIG. 9, and the white marks indicateangles of the shaft inclination according to this embodiment. Asillustrated in FIG. 11, it is understood that, in this embodiment, thenumber of significant shaft inclinations is reduced and thus the shaftinclination is suppressed as compared to the related art.

When the optical deflector 9 is driven, vibrations occur due toimbalance of the rotor unit 20. The vibrations are transferred to theboard 28, and hence the vibrating board 28 may be brought into contactwith one of the abutment portions 45 and 46 that is not originally incontact with the board 28, with the result that unusual noise isgenerated. Therefore, the abutment portions 46 and 45 may be setslightly higher than the bearing surfaces 42 a and 42 b, to therebybring both the abutment portions 45 and 46 into constant abutmentagainst the board 28. That is, the abutment portions 46 and 45 protrudefrom the bottom surface of the optical box 5 toward the board 28 in thedirection along the rotation shaft 21 higher than the bearing surfaces42 a and 42 b. Also in this case, the above-mentioned effect ofsuppressing the twist of the board 28 is maintained.

As described above, according to this embodiment, the abutment portions45 and 46 are provided, and hence, even in a case where the bearingsurface of the optical box for fixing the optical deflector 9 isinclined, the significant twist of the board 28 of the optical deflector9 can be suppressed. That is, when the optical deflector is fixed to theoptical box under a state in which the rotation shaft is inclined withrespect to the proper mounting direction, the abutment portions abutagainst the optical deflector so that the inclination can be suppressed.Thus, it is possible to suppress the distortion of the board of theoptical deflector when the optical deflector is assembled to the opticalbox, and to thereby suppress the shaft inclination. Therefore, it ispossible to stably manufacture an optical scanning apparatus 113 that issuppressed in the shaft inclination of the optical deflector 9, and tothereby prevent an image defect that may be caused by the shaftinclination in the image forming apparatus.

According to the present invention, the accuracy of the shaftinclination in a state in which the optical deflector is assembled tothe optical box can be enhanced even without the shaft inclinationcontrol function. That is, the distortion the spot-shape of the laserbeam on the photosensitive drum and the bending of the scanning line aresuppressed, and thus the image defect can be suppressed.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2012-271399, filed Dec. 12, 2012, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An optical scanning apparatus, comprising: anoptical deflector having a polygon mirror reflecting a light fluxemitted from a light source to carry out scanning, a drive unit rotatingthe polygon mirror about a rotation shaft, and a board supporting thedrive unit; an optical box to which the board is fixed; two holesprovided in the optical box, through which two screws fixing the boardare inserted, respectively; two bearing surfaces provided on the opticalbox at positions around the two holes, respectively, the two bearingsurfaces supporting the board; and an abutment portion provided on theoptical box and abutting against a portion that is prevented from beingpressed by the two screws in the board which is sandwiched between thetwo screws and the two bearing surfaces so as to be fixed to the opticalbox.
 2. An optical scanning apparatus according to claim 1, wherein,when the board of the optical deflector is fixed to the optical box, anarea of a portion of the board with which the abutment portion iscontacted is smaller than an area of a portion of the board with whicheach of the two bearing surfaces is contacted.
 3. An optical scanningapparatus according to claim 2, wherein, when the board of the opticaldeflector is fixed to the optical box, the abutment portion issubstantially brought into point contact with the board.
 4. An opticalscanning apparatus according to claim 1, further comprising a fittinghole provided in the optical box, with which the optical deflector isfitted; wherein the fitting hole is arranged on a line segmentconnecting the two holes through which the two screws are inserted,respectively.
 5. An optical scanning apparatus according to claim 1,wherein a distal end of the abutment portion protrudes toward the boardhigher than the two bearing surfaces in a direction along the rotationshaft.
 6. An optical scanning apparatus according to claim 1, whereinthe abutment portion is provided so as to protrude from the optical boxtoward the board, and is tapered so that a sectional area orthogonal tothe rotation shaft is gradually reduced toward a distal end of theabutment portion.
 7. An optical scanning apparatus according to claim 1,further comprising a protruding portion provided on the optical box soas to protrude from the optical box toward the board, wherein theprotruding portion is provided on an opposite side to the abutmentportion across a line segment connecting the two holes through which thetwo screws are inserted, respectively.
 8. An optical scanning apparatusaccording to claim 7, wherein a distal end of the protruding portionprotrudes toward the board higher than the two bearing surfaces in adirection along the rotation shaft.
 9. An optical scanning apparatusaccording to claim 7, wherein the protruding portion is tapered so thata sectional area orthogonal to the rotation shaft is gradually reducedtoward a distal end of the protruding portion.
 10. An image formingapparatus, comprising: a photosensitive member; an optical scanningapparatus irradiating and scanning the photosensitive member with light,the optical scanning apparatus having an optical deflector and anoptical box; the optical deflector including a polygon mirror reflectinga light flux emitted from a light source to carry out scanning, a driveunit rotating the polygon mirror about a rotation shaft, and a boardsupporting the drive unit; the optical box to which the board is fixed;a developing device forming a toner image by causing toner to adhereonto the photosensitive member that is irradiated with the light by theoptical scanning apparatus; a transfer device transferring the tonerimage, which is formed by the developing device, onto a recordingmaterial; two holes provided in the optical box, through which twoscrews fixing the board are inserted, respectively; two bearing surfacesprovided on the optical box at positions around the two holes,respectively, the two bearing surfaces supporting the board; and anabutment portion provided on the optical box, and abutting against aportion that is prevented from being pressed by the two screws in theboard which is sandwiched between the two screws and the two bearingsurfaces so as to be fixed to the optical box.
 11. An image formingapparatus according to claim 10, wherein, when the board is fixed to theoptical box, an area of a portion of the board with which the abutmentportion is contacted is smaller than an area of a portion of the boardwith which each of the two bearing surfaces is contacted.
 12. An imageforming apparatus according to claim 11, wherein, when the board isfixed to the optical box, the abutment portion is substantially broughtinto point contact with the board.
 13. An image forming apparatusaccording to claim 10, further comprising a fitting hole provided in theoptical box, with which the optical deflector is fitted; wherein thefitting hole is arranged on a line segment connecting the two holesthrough which the two screws are inserted, respectively.
 14. An imageforming apparatus according to claim 10, wherein a distal end of theabutment portion protrudes toward the board higher than the two bearingsurfaces in a direction along the rotation shaft.
 15. An image formingapparatus according to claim 10, wherein the abutment portion isprovided so as to protrude from the optical box toward the board, and istapered so that a sectional area orthogonal to the rotation shaft isgradually reduced toward a distal end of the abutment portion.
 16. Animage forming apparatus according to claim 10, further comprising aprotruding portion provided on the optical box so as to protrude fromthe optical box toward the board, wherein the protruding portion isprovided on an opposite side to the abutment portion across a linesegment connecting the two holes through which the two screws areinserted, respectively.
 17. An image forming apparatus according toclaim 16, wherein a distal end of the protruding portion protrudestoward the board higher than the two bearing surfaces in a directionalong the rotation shaft.
 18. An image forming apparatus according toclaim 16, wherein the protruding portion is tapered so that a sectionalarea orthogonal to the rotation shaft is gradually reduced toward adistal end of the protruding portion.
 19. A support member for anoptical scanning apparatus, which is configured to support an opticaldeflector having a polygon mirror reflecting a light flux emitted from alight source to carry out scanning, a drive unit rotating the polygonmirror about a rotation shaft, and a board supporting the drive unit,the support member comprising: two holes through which two screws fixingthe board are inserted, respectively; two bearing surfaces provided atpositions around the two holes, respectively, the two bearing surfacessupporting the board; and two abutment portions abuttable againstportions that are prevented from being pressed by the two screws in theboard, one of the two abutment portions abutting against the board whenthe board of the optical deflector is sandwiched between the two screwsand the two bearing surfaces so as to be fixed to an optical box.
 20. Asupport member for an optical scanning apparatus according to claim 19,wherein, when the board of the optical deflector is fixed, an area of aportion of the board with which the one of the two abutment portions iscontacted is smaller than an area of a portion of the board with whicheach of the two bearing surfaces is contacted.
 21. A support member foran optical scanning apparatus according to claim 20, wherein, when theboard of the optical deflector is fixed, the one of the two abutmentportions is substantially brought into point contact with the board. 22.A support member for an optical scanning apparatus according to claim19, further comprising a fitting hole with which the optical deflectoris fitted; wherein the fitting hole is arranged on a line segmentconnecting the two holes through which the two screws are inserted,respectively.
 23. A support member for an optical scanning apparatusaccording to claim 19, wherein a line segment connecting the twoabutment portions and a straight line connecting the two bearingsurfaces are substantially orthogonal to each other.
 24. A supportmember for an optical scanning apparatus according to claim 19, whereindistal ends of the two abutment portions protrude toward the boardhigher than the two bearing surfaces in a direction along the rotationshaft, respectively.
 25. A support member for an optical scanningapparatus according to claim 19, wherein each of the two abutmentportions is provided so as to protrude toward the board, and is taperedso that a sectional area orthogonal to the rotation shaft is graduallyreduced toward a distal end of the each of the two abutment portions.